Enantioselective Ring-Cleavage of meso- Epoxides with beta-Halodiisopinocampheylboranes.
PURDUE UNIV LAFAYETTE IN
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B-Halodiisopinocampheylboranes, IpcBX, in particular the bromide 1b, and the Iodide 1c, differentiate between the enantiotopic carbon-oxygen bonds of meso-epoxides to furnish 1,2-halohydrins of moderate to excellent enantiomeric purity. Thus --1R, 2R-2-bromocyclohexanol, --1R, 2R-2-iodocyclohexanol, --1R, 2R-2-iodocyclohex-4-en-1-o1 and --1R, 2R-2-bromocyclohex-4-en-1-o1 are obtained in 84, 91, 63 and 95 ee respectively from the corresponding meso-epoxides and haloborane reagents. Simple recrystallization from pentane then gives halohydrins of essentially 100 ee cis-2-butene oxide and cis-3-hexene oxide furnish the corresponding 1R, 2R-iodohydrins in 78 and 69 ee respectively. In all cases the S carbon of the meso-epoxide is selectively cleaved. This is the first example of enantioselective ring cleavage of meso-epoxides to obtain optically active 1,2-halohydrins. Keywords Asymmetric synthesis.
- Organic Chemistry
- Physical Chemistry